A Capillary Electrophoresis Sequencing Method for the Identification of Mutations in the Inverted Terminal Repeats of Adeno-Associated Virus

Abstract

Inverted terminal repeat (ITR) integrity is critical for the replication, packaging, and transduction of recombinant adeno-associated virus (rAAV), a promising gene therapy vector. Because AAV ITRs possess 70% GC content and are palindromic, they are notoriously difficult to sequence. The purpose of this work was to develop a reliable ITR sequencing method. The ITRs of two molecular clones of AAV2, pTZAAV and pAV2, were (1) sequenced directly from plasmid DNA in the presence of denaturant (direct sequencing method, DSM) or (2) first amplified in a reaction in which 7-deaza-dGTP was substituted for dGTP and the resultant amplification product sequenced (amplification sequencing method, ASM). The DSM and ASM techniques yielded clear chromatograms, read through the ITR hairpin, and revealed hitherto unreported mutations in each ITR. pTZAAV and pAV2 possess identical mutations at the upstream MscI site of the 5′ ITR (T>G, nt 2) and the downstream MscI site of the 3′ ITR (del. nt 4672–4679). The chromatograms for pAV2 also revealed that the ITRs of this construct were arranged in a FLOP/FLOP orientation. In addition, the DSM was successfully employed to recover ITR–chromosomal junction sequences from a variety of rAAV-transduced tissue types. Both the DSM and ASM can be employed to sequence through the AAV ITR hairpin, and both techniques reliably detect mutations in the ITR. Because the DSM and ASM offer a way to verify ITR integrity, they constitute powerful tools for the process development of rAAV gene therapy.

Mroske and colleagues describe two methods for screening the sequence integrity of the palindromic, high-GC-containing internal terminal repeats (ITRs) of AAV. They show that direct sequencing of each ITR on AAV plasmids (direct sequencing method), and sequencing of an amplification product containing a complete 7-deaza-dGTP substitution for dGTP (amplification sequencing method) can be used to produce noise-free chromatograms that reveal mutations in concordance with the respective restriction patterns.

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